Fuel cells are well known and are commonly used to produce electrical current from hydrogen containing reducing fluid fuel and oxygen containing oxidant reactant streams to produce electrical power. In fuel cells of the prior art, it is well known that fuel is often produced by a reformer and the resulting hydrogen rich fuel flows from the reformer through a fuel inlet line into an anode flow field of the fuel cell. As is well known an oxygen rich reactant simultaneously flows through a cathode flow field of the fuel cell to produce electricity. Unfortunately, known fuels for fuel cells, such as reformate fuels from reformers, frequently contain contaminants especially ammonia. The presence of ammonia in the fuel stream is detrimental to the performance of the fuel cell.
It is understood that ammonia is a common byproduct of the reforming process and although the reforming process is designed to minimize formation of ammonia, it is common that low levels of ammonia are present in the reformate fuel. Nitrogen present in a hydrogen rich fuel reacts with hydrogen in common steam reformers, which typically utilize conventional nickel catalysts, to form ammonia in a concentration range of parts per million. The ammonia causes performance degradation of the fuel cell when introduced to either a proton exchange membrane fuel cell (“PEMFC”) or a phosphoric acid fuel cell (“PAFC”). Fuel specifications for a known PAFC power plant require a maximum allowable nitrogen concentration in the natural gas of 4% to prevent degradation of the fuel cell due to ammonia formation. This fuel specification places a serious limitation on the use of fuel cells in areas of the world where natural gas includes a nitrogen content in excess of the fuel specification. Additionally, in the case of auto thermal or partial oxidation reformers, nitrogen can also be introduced when air is used as the oxygen source for the reforming process.
Many efforts have been undertaken to remove ammonia and other contaminants from fuel streams of fuel cells. For example, U.S. Pat. No. 4,259,302 to Katz et al. discloses use of a regenerable scrubber for removing ammonia from a fuel cell fuel stream. Ammonia gas is scrubbed from the fuel stream in a bed of support material soaked with acid. In Katz et al., the preferred acid is phosphoric acid and the preferred support material is carbon in the form of porous carbon particles or pellets. As disclosed in Katz, when an ammonia contaminant passes through a scrubber having phosphoric acid, the ammonia is absorbed on the support material and reacts with the phosphoric acid as follows:H3PO4+NH3→(NH4)H2PO4  (Equation 1)
As is apparent, in removing the ammonia contaminant, the phosphoric acid is converted to ammonium dyhydrogen phosphate. When approximately 50 percent of the phosphoric is converted, there is a risk of breakthrough of the ammonia out of the scrubber and into the fuel stream passing into the fuel cell. Consequently, the capacity for removal of ammonia by known scrubbers is determined by an anticipated level of ammonia in the fuel stream and the operational duration of the fuel cell prior to service or replacement of the scrubber.
For example, U.S. Pat. No. 5,792,572 to Foley et al. shows another effort at minimizing ammonia contamination wherein a scrubber contains phosphoric acid absorbed onto porous carbon pellets. The scrubber of Foley et al. is utilized to both remove ammonia from the fuel stream and also to add acid to the fuel cell in a controlled manner. However, because of the described conversion of phosphoric acid within the scrubber, in the Foley et al. system, a scrubber capable of providing acceptable ammonia removal for five to ten years without replacement must be unacceptably large, bulky and excessively costly.
More recently U.S. Pat. No. 6,376,114, that issued on Apr. 23, 2002 to Bonville, Jr. et al., discloses another elaborate system for removing ammonia and other contaminants from reformate fuel. The system of Bonville, Jr. et al., includes alternatively a disposable ammonia scrubber, an ammonia scrubbing cool water bed and an ammonia stripping warm water bed, a pair of first and second regenerable scrubbers, or a single regenerable scrubber. Again, while effective the Bonville, Jr. et al. system includes elaborate and costly components that require a high level of maintenance to operate the system. The aforesaid three patents are owned by the assignee of all rights in the present disclosure.
For known scrubbers that include acid absorbed on a support material, a standard volume efficiency is between 0.10 to 0.30 units of volume of acid per unit of internal volume of the scrubber depending upon a particular carbon and shape of the carbon. Granular materials have higher packing efficiencies at an expense of a lower pressure drop. In contrast, cylindrical materials have a lower pressure drop at an expense of lower packing efficiencies. As described above breakthrough of the ammonia out of the scrubber and into the fuel stream typically occurs after only 50 percent of the conversion of the acid because of concentration gradients within the particular support material utilized within the scrubber. Passing of the gaseous fuel stream through the support material causes disproportionate contact with acid adjacent fuel stream passages within the support material. Acid concentrated within the support material and away from such fuel stream passages may experience little or no contact with the fuel stream. Therefore, to operate efficiently known scrubbers require a high volume of acid per unit of internal volume of the support material, and a corresponding high volume of support material in a large scrubber.
Other ammonia and related contaminant removal systems for fuel cells are known in the art. However, none of these provide for efficiently removing ammonia with minimal costs and minimal maintenance requirements. Most known ammonia contaminant removal systems require large components for processing a high volume of fluids, or require high frequency removal and replacement of expensive, contaminated filters and/or ion beds, etc.
Consequently, there is a need for a ammonia removal system for a fuel stream of a fuel cell that significantly reduces overall system size and that may be operated efficiently for long periods of time without high frequency maintenance.